Agent Skills: lisa-tracker-validate

Vendor-neutral wrapper for the pre-write quality gate. Reads the required `tracker` from .lisa.config.json and dispatches to lisa-jira-validate-ticket, lisa-github-validate-issue, or lisa-linear-validate-issue. Read-only — never writes to any tracker. Used by tracker-write Phase 5.5 (pre-write gate), tracker-verify (post-write checks), and the *-to-tracker dry-run paths. Output is structured PASS/FAIL per gate so callers can parse it.